Evaluation of the Exchange Stiffness Constants of Itinerant Magnets at Finite Temperatures from the First-Principles Calculations

We attempted to evaluate the exchange stiffness constant, A(T) of fcc-Fe_0.2Ni_0.8 and L1₀-type FePt at finite temperatures employing the first-principles calculation. The spin fluctuation at a finite temperature was handled using the coherent potential approximation under the disordered-local-moment picture, and the A(T) was evaluated based on the free energy of spin spiral structure created in the effective medium representing the fluctuating spins. For fcc-Fe_0.2Ni_0.8, calculated A(0) was approximately 10 meV/Å. With increasing temperature, A(T) exhibits a linear-like decrease and becomes approximately 7 meV/Å at around room temperature which is in the range of experimentally expected value. This behavior somewhat deviates from the theoretical prediction considering a magnon–magnon scattering in the Heisenberg model. For FePt, we found a considerable anisotropy between A₀₀₁(T) and A₁₀₀(T) for spiral wave vectors along the directions of 〈001〉 and 〈100〉, respectively.